In the recent semiconductor processing technology, a challenge to higher integration of large-scale integrated circuits places an increasing demand for miniaturization of circuit patterns. There are increasing demands for further reduction in size of circuit-constructing wiring patterns and for miniaturization of contact hole patterns for cell-constructing inter-layer connections. As a consequence, in the manufacture of circuit pattern-written photomasks for use in the photolithography of forming such wiring patterns and contact hole patterns, a technique capable of accurately writing finer circuit patterns is needed to meet the miniaturization demand.
In order to form a higher accuracy photomask pattern on a photomask substrate, it is of first priority to form a high accuracy resist pattern on a photomask blank. Since the photolithography carries out reduction projection in actually processing semiconductor substrates, the photomask pattern has a size of about 4 times the actually necessary pattern size, but an accuracy which is not loosened accordingly. The photomask serving as an original is rather required to have an accuracy which is higher than the pattern accuracy following exposure.
Further, in the currently prevailing lithography, a circuit pattern to be written has a size far smaller than the wavelength of light used. If a photomask pattern which is a mere 4-time magnification of the circuit feature is used, a shape corresponding to the photomask pattern is not transferred to the resist film due to influences such as optical interference occurring in the actual photolithography operation. To mitigate these influences, in some cases, the photomask pattern must be designed to a shape which is more complex than the actual circuit pattern, i.e., a shape to which the so-called optical proximity correction (OPC) is applied. Thus, at the present, the lithography technology for obtaining photomask patterns also requires a higher accuracy processing method. The lithographic performance is sometimes represented by a maximum resolution. As to the resolution limit, the lithography involved in the photomask processing step is required to have a maximum resolution accuracy which is equal to or greater than the resolution limit necessary for the photolithography used in a semiconductor processing step using a photomask.
A photomask pattern is generally formed by forming a photoresist film on a photomask blank having a light-shielding film on a transparent substrate, writing a pattern using electron beam, and developing to form a resist pattern. Using the resulting resist pattern as an etch mask, the light-shielding film is etched into a light-shield pattern. In an attempt to miniaturize the light-shield pattern, if processing is carried out while maintaining the thickness of the resist film at the same level as in the art prior to the miniaturization, the ratio of film thickness to pattern width, known as aspect ratio, becomes higher. As a result, the resist pattern profile is degraded, preventing effective pattern transfer, and in some cases, there occurs resist pattern collapse or stripping. Therefore, the thickness of resist film must be reduced to enable miniaturization.
As to the light-shielding film material to be etched through the resist pattern as etch mask, a number of materials are known in the art. Among others, chromium compound films are used in practice because many teachings about etching are available and their processing has been established as the standard process. For example, a photomask blank having a light-shielding film composed of a chromium compound suited for ArF excimer laser lithography is disclosed in JP-A 2003-195479. Specifically a chromium compound film having a thickness of 50 to 77 nm is described.
A typical dry etching process for chromium base films such as chromium compound films is oxygen-containing chlorine gas dry etching, which has a certain etching ability relative to organic film. Thus, when etching is conducted through a thinner resist film in order to transfer a finer size pattern for the above-described reason, the resist film can be damaged during etching. It is then difficult to transfer the resist pattern accurately. To meet both the requirements of miniaturization and accuracy, it becomes necessary to investigate the light-shielding material again so as to facilitate the processing of light-shielding film, rather than the current trend relying solely on resist performance improvement.
For example, JP-A 2006-78807 discloses a light-shielding film including at least one layer of a material mainly containing silicon and a transition metal wherein an atomic ratio of silicon:metal is 4-15:1. The light-shielding film has improved light shielding performance and ease of processing and is suited for the ArF lithography. Also JP-A 2007-241060 discloses a photomask blank including a light-shielding film containing silicon and a transition metal and a thin film of chromium base material as a hard mask film, with the advantage of high accuracy processing.
As described above, a light-shielding film which can be processed under conditions which cause less damages to the resist pattern is necessary in order to form a finer size pattern accurately. In the case of a photomask blank including a light-shielding film containing silicon and a transition metal as elements for providing a transmittance reducing function and optionally low atomic weight elements such as nitrogen and oxygen and a chromium base hard mask film, proposed in JP-A 2007-241060, one effective means for reducing the load to the resist is by reducing the thickness of the light-shielding film itself as well as the thickness of the hard mask film. In this case, particularly on the light-shielding film side, the concentration of low atomic weight elements added to the material such as nitrogen and oxygen is minimized in order to derive a better light shielding effect from a thin film. That is, a so-called highly metallic film is used as the light-shielding film.
On use in the exposure tool, the photomask is mounted in the exposure tool such that the pattern-bearing surface may face the object to be exposed (e.g., silicon wafer). Upon exposure, exposure light is incident on the surface of the transparent substrate opposite to the pattern-bearing surface, i.e., substrate surface, and transmitted by the transparent substrate. The component of light that has passed the region of the pattern-bearing surface where the light-shielding pattern is absent reaches the resist film whereby the resist film is exposed to light patternwise.
At this point, it is known that a ghost pattern is created by the mechanism that the exposure light is reflected by the surface of a substrate to be exposed (e.g., silicon wafer) which is coated with the resist film and the reflected light is reflected again by the light-shielding pattern of the photomask and reaches the resist film again. Thus an antireflective coating is generally formed on the light-shielding film of the photomask for controlling the reflectance of the photomask.
On the other hand, it is also contemplated that a component of incident light entering the transparent substrate surface is reflected by the interface between the light-shielding film and the transparent substrate, and the reflected light is reflected again by the surface of the transparent substrate (interface between the transparent substrate and the surrounding atmosphere), passes through the light-shielding pattern-free region and reaches the resist film. For example, when the light-shielding film is entirely formed as an all metal film completely free of light elements in order to minimize the thickness of light-shielding film, the reflectance of exposure light of wavelength 193 nm at the interface between the light-shielding film and the transparent substrate reaches a value in excess of 50%, with the risk of creating a ghost pattern. Particularly when the exposure tool uses an oblique incidence illumination system such as modified illumination, there is a strong possibility that the exposure light reflected again will pass through the light-shielding pattern-free region, which makes the problem more serious. Then WO 2008/139904 proposes that an antireflective coating capable of attenuating reflected light by utilizing multiple reflection is formed on the back surface side to reduce reflectance.
However, this antireflective film must be designed such that the film thickness multiplied by refractive index may approximate to a quarter of the exposure wavelength and it may have a certain transmittance, before the antireflective film can fully exert its function. In order that the antireflective film approach attain a necessary degree of light shielding and a low reflectance, the overall film must be thicker. Then the etching time during pattern transfer is accordingly increased. As a result, the resist pattern or hard mask pattern serving as the etch mask during etching process must be thick, which is disadvantageous in forming a finer size pattern at a high accuracy. Further, when an antireflective layer is formed, a difference in etching rate during dry etching arises from the compositional difference thereof from the light-shielding film, resulting in the light-shielding film pattern being stepped or distorted on its sidewall.